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NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM NCHRP SYNTHESIS 425 Systematic, well-designed research provides the most effective Project 20-05 (Topic 42-07) approach to the solution of many problems facing highway administra- ISSN 0547-5570 tors and engineers. Often, highway problems are of local interest and ISBN 978-0-309-22346-1 can best be studied by highway departments individually or in coop- Library of Congress Control No. 2011943710 eration with their state universities and others. However, the accelerat- ing growth of highway transportation develops increasingly complex problems of wide interest to highway authorities. These problems are 2012 National Academy of Sciences. All rights reserved. best studied through a coordinated program of cooperative research. In recognition of these needs, the highway administrators of the American Association of State Highway and Transportation Officials COPYRIGHT INFORMATION initiated in 1962 an objective national highway research program Authors herein are responsible for the authenticity of their manuscripts employing modern scientific techniques. This program is supported on a and for obtaining written permissions from publishers or persons who continuing basis by funds from participating member states of the Asso- own the copyright to any previously published or copyrighted material ciation and it receives the full cooperation and support of the Federal used herein. Highway Administration, United States Department of Transportation. Cooperative Research Programs (CRP) grants permission to repro- The Transportation Research Board of the National Research Coun- duce material in this publication for classroom and not-for-profit pur- cil was requested by the Association to administer the research pro- poses. Permission is given with the understanding that none of the gram because of the Board's recognized objectivity and understanding material will be used to imply TRB, AASHTO, FAA, FHWA, FMSCA, of modern research practices. The Board is uniquely suited for this FTA, or Transit development Corporation endorsement of a particular purpose as it maintains an extensive committee structure from which product, method, or practice. It is expected that those reproducing the authorities on any highway transportation subject may be drawn; it material in this document for educational and not-for-profit uses will possesses avenues of communication and cooperation with federal, give appropriate acknowledgment of the source of any development or state, and local governmental agencies, universities, and industry; its reproduced material. For other uses of the material, request permission relationship to the National Research Council is an insurance of objec- from CRP. tivity; it maintains a full-time research correlation staff of specialists in highway transportation matters to bring the findings of research directly to those who are in a position to use them. NOTICE The program is developed on the basis of research needs identified by chief administrators of the highway and transportation departments The project that is the subject of this report was a part of the National and by committees of AASHTO. Each year, specific areas of research Co-operative Highway Research Program conducted by the Transpor- needs to be included in the program are proposed to the National tation Research Board with the approval of the Governing Board of Research Council and the Board by the American Association of State the National Research Council. Such approval reflects the Governing Highway and Transportation Officials. Research projects to fulfill Board's judgment that the program concerned is of national importance these needs are defined by the Board, and qualified research agencies and appropriate with respect to both the purposes and resources of the are selected from those that have submitted proposals. Administration National Research Council. and surveillance of research contracts are the responsibilities of the The members of the technical committee selected to monitor this National Research Council and the Transportation Research Board. project and to review this report were chosen for recognized scholarly The needs for highway research are many, and the National Coop- competence and with due consideration for the balance of disciplines erative Highway Research Program can make significant contributions appropriate to the project. The opinions and conclusions expressed or to the solution of highway transportation problems of mutual concern implied are those of the research agency that performed the research, to many responsible groups. The program, however, is intended to and, while they have been accepted as appropriate by the technical com- complement rather than to substitute for or duplicate other highway mittee, they are not necessarily those of the Transportation Research research programs. Board, the National Research Council, the American Association of State Highway and Transportation Officials, or the Federal Highway Administration of the U.S. Department of Transportation. Each report is reviewed and accepted for publication by the technical committee according to procedures established and monitored by the Transportation Research Board Executive Committee and the Govern- ing Board of the National Research Council. Published reports of the NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM are available from: Transportation Research Board Business Office NOTE: The Transportation Research Board of the National Acad- 500 Fifth Street, NW emies, the National Research Council, the Federal Highway Adminis- Washington, DC 20001 tration, the American Association of State Highway and Transporta- tion Officials, and the individual states participating in the National and can be ordered through the Internet at: Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturers' names appear herein solely because they are considered essential to the object of this report. Printed in the United States of America